Distributive Nd-to-Yb Energy Transfer within Pure [YbNdYb] Heterometallic Molecules

Facile access to site-selective hetero-lanthanide molecules will open new avenues in the search of novel photophysical phenomena based on Ln-to-Ln′ energy transfer (ET). This challenge demands strategies to segregate efficiently different Ln metal ions among different positions in a molecule. We report here the one-step synthesis and structure of a pure [YbNdYb] (1) coordination complex featuring short Yb···Nd distances, ideal to investigate a potential distributive (i.e., from one donor to two acceptors) intramolecular ET from one Nd3+ ion to two Yb3+ centers within a well-characterized molecule. The difference in ionic radius is the mechanism allowing to allocate selectively both types of metal ion within the molecular structure, exploited with the simultaneous use of two β-diketone-type ligands. To assist the photophysical investigation of this heterometallic species, the analogues [YbLaYb] (2) and [LuNdLu] (3) have also been prepared. Sensitization of Yb3+ and Nd3+ in the last two complexes, respectively, was observed, with remarkably long decay times, facilitating the determination of the Nd-to-Yb ET within the [YbNdYb] composite. This ET was demonstrated by comparing the emission of iso-absorbant solutions of 1, 2, and 3 and through lifetime determinations in solution and solid state. The comparatively high efficiency of this process corroborates the facilitating effect of having two acceptors for the nonradiative decay of Nd3+ created within the [YbNdYb] molecule.


Other Physical Measurements
Elemental analyses were performed with a Perkin-Elmer Series II CHNS/O Analyzer 2400 (C, H, N) at the Servei de Microanàlisi of CSIC, Barcelona. IR spectra were recorded as KBr pellet samples on a Nicolet 5700 FTIR spectrometer.

Mass Spectrometry
Positive-ion ESI mass spectrometry experiments were performed by using a LC/MSD-TOF (Agilent Technologies) with a dual source equipped with a lock spray for internal reference introduction, at the Unitat d'Espectrometria de Masses from the Universitat de Barcelona. Experimental parameters: capillary voltage 4 kV, gas temperature 325°C, nebulizing gas pressure 103.42 kPa, drying gas flow 7.0 L min-1 and fragmentor voltage 175-250 V. Internal reference masses were m/z 121.05087 (purine) or 922.00979 (HP-0921). Crystals of 1, 2 or 3 were dissolved in mixtures of MeOH with the minimal amount of DMSO and introduced into the source by using a HPLC system (Agilent 110) with a mixture of H2O/CH3CN (1:1) as the eluent (200 μL min -1 ). As observed previously for related clusters, the ionization caused the removal of both pyridine and water ligands from the complexes. For each complex, moieties related exclusively to the expected [LnLn'Ln] metal distribution were observed. Moreover, no signals for other metallic compositions were detected, thus evidencing not only the realization of the trinuclear heterometallic compound but also its robustness and exclusiveness in solution.

Single-crystal X-ray diffraction
Data for compound [NdYb2(LA)2(LB)2(py)(H2O)2](NO3) (1) were obtained at 100 K with a Bruker APEX II QUAZAR diffractometer equipped with a microfocus multilayer monochromator with Mo Kα (λ = 0.71073 Å). Data for compound [LaYb2(LA)2(LB)2(py)(H2O)2](NO3) (2) were collected at 100 K at Beamline 12.2.1 of the Advanced Light Source (Berkeley, USA), on a Bruker D8 diffractometer equipped with a PHOTON II detector and using silicon (111) monochromated synchrotron radiation (λ = 0.7288 Å). Data for compound [NdLu2(LA)2(LB)2(py)(H2O)2](NO3) (3) were acquired at 100 K on the BL13-XALOC beamline 3 of the ALBA synchrotron (λ = 0.72932 Å). Data reduction and absorption corrections for 1 and 2 were performed with respectively SAINT and SADABS. 4 Data reduction for compound 3 were done with autoproc package 5 and XDS. 6 All structures were solved with Olex2 7 (1) or SHELXT 8 (2 and 3) and refined by full-matrix least-squares on F 2 with SHELXL. 9 In the structures of 2 and 3, a portion of the lattice solvent molecules were too diffuse/disordered to be modelled satisfactorily. The corresponding void spaces were thus analysed and taken into account with PLATON/SQUEEZE 10 (2) or Olex2 (3), the formula reflecting the squeezed/masked content. The lanthanide sites composition in the model is supported by the worse agreement factors and unrealistic relative Ueq values observed for any other combination of the lanthanide sites.
All details can be found in CCDC 2209574-2209575-2209576 (1-2-3) which contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via https://summary.ccdc.cam.ac.uk/structure-summary-form.
Crystallographic and refinement parameters are summarized in Table S1, while Tables S2 and S3 provide Ln-N bond lengths and details of hydrogen bonds in the structures of 1, 2, and 3.

Photo physical Studies
The luminescence spectra were measured using a Horiba-Jobin Yvon Fluorolog-3® spectrofluorimeter, equipped with a three slit double grating excitation monochromator with dispersions of 2.1 nm/mm (1200 grooves/mm) and a single grating iHR320 monochromator for the emission with a dispersion of 20 nm/mm (150 grooves/mm). The steady-state luminescence was excited by unpolarized light from a 450 W xenon CW lamp and detected at an angle of 90° through a FGL850 filter by a Symphony® II CCD detector. Spectra were reference corrected for both the excitation source light intensity variation (lamp and grating) and the emission spectral response (detector and grating).